Sky and Telescope - February 2017 - 20

Black Holes, Part II
W STARS AND BLACK HOLES This figure tracks the growth of black
holes and of galaxies (manifested as star formation) over cosmic time. At
first glance, the graph appears to show that black holes and galaxies coevolve. But while black hole accretion and star formation rates do track
each other closely in the last 10 billion years, that might arise if they're
controlled by the same thing - for example, the gradual decline of fuel
as the universe's cold gas supply is used up. Another explanation is that,
if both black holes and galaxies grow early on thanks to galaxy mergers, then the rates might decline in recent cosmic times because galaxy
mergers are increasingly rare as the universe expands.

10 -2
Black hole accretion rate,
averaged over cosmic
volume and scaled up
by 5000x

10 -3

Extrapolation
Star-formation rate
measurements

0.5

0.9

2

6

Time since Big Bang (billion years)

13.8
(Today)

How We "Weigh" Black Holes
◗ Obviously astronomers can't put a black hole
on a bathroom scale. (It's impolite.) So to measure
the mass of one of these objects, observers clock
the speed of stars or gas whirling around it in a
galaxy's core. These velocities depend on the mass
of the black hole that the stars or clouds are orbiting. When observers can also see how far away this
stuff is from the black hole, they can then directly
measure the beast's mass.
When it's impossible to see how large these
orbits are directly, astronomers turn to the accreting
material's glow. Active black holes are notorious for
their flickering. The delay between flickers corresponds to how long light took to travel from one
side to the other of the accreting gas - and since
light travels at a finite speed, the travel time tells us
the distance crossed. That distance, plus the gas's
orbital speed, corresponds to how massive the
black hole is.
When all else fails, astronomers estimate the
mass based on the feeding black hole's brightness.

20

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

ton University). Astronomers immediately started speculating
that galaxies and black holes grow in lockstep. "It had to be
some kind of feedback loop between the black hole and the galaxy," she says, describing what scientists thought at the time.
Many suspected that the black hole dominated the relationship. If so, these little spacetime monsters controlled not
just unlucky, passing stars but galaxy formation across the
universe, serving as invisible masterminds in the development of cosmic structure. Black holes were suddenly the most
important object in the cosmos. When I wrote my master's
thesis on these objects in 2010, one astronomer told me that
"understanding the whole history of the universe is locked up
in understanding black holes."
But with more data, astronomers are realizing that the tale
isn't so magically simple. The saga may not have the black
hole as its all-powerful hero. Instead, the hole might just be
along for the ride.

Messed-up M-sigma
At first, astronomers thought all galaxies obeyed M-sigma
and the other correlations. But soon they realized that wasn't
the case. True, the correlations did hold in typical elliptical
galaxies, those big bulbous balls of old stars. But the trends
show up only weakly, if at all, in disk galaxies. In such systems, none of the galaxies' properties - their total mass in
stars, their bulge mass, the range of star speeds, or the mass
of the dark matter clouds they sit in - closely aligns with the
central black hole's mass.
If the M-sigma rule applied to every galaxy, then disk
galaxies should have black holes much beefier than they do.
Instead, several astronomers have seen a "lightweight" trend
in recent years, including Greene's team.
"The only population left where there is a tight relationship between the black hole and the galaxy are these typical
ellipticals," Greene says. Even the most massive ellipticals
don't follow it well, she adds.
But spreading the message that there's no universal, tight
coevolution has taken time. "We're terribly human people, and
the psychology kind of took over," says John Kormendy (University of Texas at Austin), one of the first to suggest a relationship between black holes and their galaxies - and, with Luis
Ho (Peking University, China), one of the first to raise the
red flag against the lockstep growth many astronomers came
to believe in. "Scientists get very sure of the things that they
think they're very sure of. And sometimes they've been wrong
- and when they are, it's a hell of a job to change the folklore."

CHART: S&T: LE A H TISCIONE, SOURCE J. KOR MENDY A ND L. HO / ANNUAL R E VIE W OF
ASTR ONOMY AND ASTR OPHYSICS 2013; ILLUSTR ATION: S&T / G REGG DINDER M A N

Star-formation rate (solar masses
per year per cubic megaparsec)

10 -1



Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

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